This invention relates to devices to frequency variable devices that incorporate a piezoelectric thin film resonator.
Crystal media capable of coupling mechanical and electrical signals have found application in many useful devices, e.g. producing a voltage in response to mechanical stress or vice versa. Piezoelectric crystals are a well-known group of such materials. Examples of materials which may have piezoelectric properties include aluminum nitride (AlN), zinc oxide (ZnO), Quartz (SiO2) etc. Piezoelectric materials may be characterized as having a permanent fixed charge in the crystal structure thereof, which may be thought of as electrical domains in alignment. Conventional piezoelectric materials suffer from a number of drawbacks which limit their application in certain types of devices. Since the electrical orientation of a piezoelectric material is fixed, it cannot be turned off and a switch is required to disconnect the device from the rest of the circuit. Such switches are not desirable in RF circuits, since the use of switch networks entails energy loss, increased device complexity, and cost. In an acoustic resonator device, where it is desired to use an electric field to alter acoustic wave propagation in the material, applying an electric field only changes the electromechanical coupling (and hence the acoustic velocity) by a modest amount in conventional piezoelectric materials because the materials have a permanent electric field. Consequently, it is not efficient to use an external electric field to significantly tune the acoustic velocity and electromechanical coupling coefficient of these resonators. A subset group of piezoelectric crystal media, generally referred to as displacive ferroelectric materials, exhibits induced piezoelectricity with the application of an external stress (such as a DC bias voltage). Displacive ferroelectric materials differ from piezoelectric materials in that these materials do not exhibit piezoelectric behavior unless an external stress is applied to induce such behavior in the material. This is because, unlike conventional piezoelectric materials (which are used in a variety of device contexts such as, for example, resonators), these displacive ferroelectric materials have no net displacement of charge in their crystal structure in their paraelectric state.
In displacive ferroelectric materials, an applied DC bias causes an incremental change in the strain of these materials by creating non-centrosymmetry in the crystal structure that can be interpreted as a change in the stiffness matrix for the material. This change in the stiffness (or elastic constant) of materials also induces the change of acoustic velocity inside of materials by the following known expression:
Vac (acoustic velocity)=SQRT (c33/ρ); where ρ is the density of the material and c33 is the elastic constant. Rosenbaum, J. F., Bulk Acoustic Wave Theory and Devices, (1988).
The acoustic path length is the distance traversed by an acoustic wave of a specified velocity through the material. Eventually, the applied DC bias voltage causes a change in the acoustic path length of the material.
The aforementioned properties may be found in perovskite crystals having the general chemical formula ABO3, for example SrTiO3 as shown in FIG. 1A. Due to the mismatch in the sizes of the two cations A and B, small distortions in the lattice occur to minimize the lattice energy. If these distortions are asymmetric, a small residual charge displacement in the crystal structure will result. In the piezoelectric state, the crystal is formed in such a way that the charge displacement is permanent. In the paraelectric state, on the other hand, there is no net displacement of charge in the absence of a DC bias; the centers of positive and negative charge coincide at the center 21 of the crystal, as shown in FIG. 1B. When a DC bias is applied (e.g. an external electric field), the positive and negative charge centers separate slightly, as shown in FIG. 1C. Accordingly, these displacive ferroelectric materials in the paraelectric state do not store a permanent charge. Application of an external stress (such as a DC electric field) generates a static charge displacement.
However, not every displacive ferroelectric material is optimal for bias-induced acoustic path and piezoelectric tuning. For example, Gevorgian and Vorobiev, J. Appl. Phys., Vol. 99, p. 124112 (2006) describes a thin film bulk acoustic resonator (TFBAR) using SrTiO3 or Ba0.25Sr0.75TiO3 as the paraelectric that is biased to obtain piezoelectric behavior. Further, the same group in Berge et al. (IEEE Microwave and Wireless Components Lett., 17(9):655-657, September 2007) extend the previous work to solidly mounted resonators (SMR) using BaTiO3 or Ba0.25Sr0.75TiO3 as the tunable piezoelectric layer. Another group, Saddik et al., Appl. Phys. Lett., Vol. 91, 043501 (2007) describes a SMR using SrTiO3 as the paraelectric material that is biased to obtain piezoelectric behavior. International Patent #WO 2006/004470 A1, published on 12 Jan. 2006, describes the tunable FBAR structure described above in Gevorgian and Vorobiev. U.S. Pat. No. 6,747,529, describes TFBAR and SMR structures with the epitaxial ferroelectric BaTiO3 as the tunable piezoelectric layer. BaTiO3 is a material that is piezoelectric even without an externally applied bias. These references use electrical bias and tuning to place a paraelectric material into piezoelectric operation. However, this approach is subject to several flaws and drawbacks:                1) Bandwidth. The performance of such devices is limited by the achievable bandwidth of the tunable material. Typically, it is seen that this bandwidth is of the order of 1-3% in TFBARs.        2) Nonlinearity. The tuning behavior of these materials in inherently nonlinear; when they are subject to an AC voltage, the resonance frequency of the device changes during operation. This can cause undesirable overtones to be generated in the output of the device.        3) DC Bias Voltage. Even though small bias voltages will induce piezoelectric behavior, the resulting small electromechanical coupling coefficient (k2) of the resonator will be practically unusable. To obtain useful values of k2, it would become necessary to use large bias voltages, which are difficult and expensive to generate. High bias voltages can also significantly reduce the lifetime of devices.        
Another patent, U.S. Pat. No. 6,534,900 describes TFBAR and SMR structures that use electrostrictive polymers and non-polar ceramics as their electroactive tuning layer. However, these materials suffer from the same drawbacks of other paraelectric materials noted above. Additionally, polymeric materials can suffer from significant hysteresis.
It should be noted that in a paraelectric state, the sign of the coupling between the incremental voltage and the strain may be reversed by reversing the sign of the bias voltage. This property clearly distinguishes paraelectric materials from piezoelectric materials. It follows that in paraelectric state, the strength of the coupling between mechanical and electrical signals can be modulated, reduced to zero, or even have its sign reversed. Since paraelectric materials do not have a net displacement of charge in the absence of an applied stress (or applied electric field), they tend to be more rugged than piezoelectric materials in high-temperature or high-power environments.
Despite the above noted deficiencies in the performance of resonator devices that utilize such materials, it remains desirable to exploit the distinct properties of paraelectric materials as electromechanical and/or electro-acoustic transmissive layers.